Method for producing pressed products from fiber-containing raw materials

ABSTRACT

A method produces pressed products from fiber-containing raw materials such as wood, straw, peat or suchlike, which are also referred to as secondary raw materials. 
     In order to produce well pressed products, fiber-containing raw materials are mixed with water, vapor and, if need be, further additives while heating the raw materials and additives, the mixture being held at high temperature for a lengthy period of time before pressing.

BACKGROUND OF THE INVENTION

The invention relates to a method for producing pressed products from fiber-containing raw materials such as wood, straw, peat or suchlike, which are also referred to as secondary raw materials.

The invention also relates to a device for performing such a method.

Pellet mills or feed pellet mills or pellet presses have been used in practice for many years in the feedstuff and foodstuff industry and also in the non-food sector. Grainy, powdery and/or pasty materials with the most varied moisture contents and compositions are pressed.

They comprise pressing moulds and strippers in order to produce cubes or pellets in the desired size. Such a pellet mill is disclosed for example in EP-A-371519. The material to be pressed passes from a feed hopper into a conveying and metering apparatus and from there into a mixer. Both apparatuses are disposed at the upper side of the housing of the actual press, whereby the outlet of the mixer conveys into a bent feed box (and into a mould cover), which is disposed thereunder but is not rigidly connected to the mixer. The material passes from the mould cover into the pressing mould.

One pressing mould is fixedly connected to a rotating press moulding holder. A pulley carrier with a roller bar fixed thereto is also disposed fixedly. Two rollers are disposed adjustably by means of a piston-cylinder unit, so that different gaps and pressing pressures can be adjusted on the mould die.

Another pelletising press is shown by EP-B-489046, wherein the mould and at least one press roller can be driven at variable speeds and as a function of the gap height.

Various solutions are known for fixing the moulds in a pellet mill. The initially mentioned fixed arrangement, which involves simply screwing down the mould, is cost-effective, but necessitates a time-consuming mould replacement. Also known, however, are solutions for a rapid replacement, e.g. according to U.S. Pat. No. 4,979,887, wherein for example the individual clamping segments are actuated by short-stroke hydraulic cylinders at the periphery of a wear ring. The hydraulic cylinders are connected to an external hydraulic pump for the mould replacement.

According to DE-OS 2108326, it is also known, for the purpose of better assembly, to hold the clamping segments by means of spring elements in the position that clamps the die, adjusting means being provided which release the clamping segments against the action of the spring. As a result of the spring tensioning forces being overcome, the die is intended to be easily released from the column or can be fixed again. Hydraulic winches are used as the adjusting means. A similar solution is shown by DE-OS 2756647, wherein a conical ring is used for fixing the die.

Mixing units are known for example from EP-B-231764 and EP-B-610789. Here, a metering device is incorporated upstream of such a mixer and a pressing mould or an expander is in turn fed from the mixer. The mixing unit is provided with a heating jacket and contains a rotating shaft, which is fitted with mixing and conveying elements.

Such units are either tailor-made for use in pressing or expander plant or can be used solely in isolation, since the adaptation cost is high.

Pressing and expander plant are used primarily for the production of foodstuff or feedstuff, although the production of pellets from wood or waste from grinding grain, for example from wheat bran, is however also known. DE-A-10205105 discloses a method for producing pellets from wood, wherein wood material with a pre-adjusted moisture content is pressed in a pelletising press, material with higher moisture content being pre-pelletised with the generation of heat and moisture subsequently being conveyed away. It is known from DE-A-10010414 to mix wood fibers or suchlike with up to 50 wt. % of a thermosetting resin and to extrude the mixture.

SUMMARY OF THE INVENTION

The problem underlying the invention is to develop a method for producing pressed products from fiber-containing raw materials, which for example enables the production of pellets from wood, straw, peat or paper, plastic mixtures, or any kind of secondary raw materials which can be pelletised for energy production.

The method is characterised by a lengthy dwell time (long-time conditioning) at high temperature after the mixing-in of water, vapour and possibly other additives. The dwell period is prior to the pressing.

Long-time conditioning is understood to mean the process time or dwell time lying above the normal mixing time, possibly with two mixers incorporated one after the other, each mixer approx. 15-20 seconds. There is also an additional dwell time in a buffer depot.

The conditioning temperature can amount to up to 200° C., a range up to 150° C. being preferred.

The conditioning can also take place under excess pressure (high-pressure conditioning). In order to be able to reach dwell times up to several hours, the dwell containers/maturing containers are preferably insulated.

Further additives are for example fat or lignin.

The device comprises an intermediate container disposed downstream of a mixer, in which intermediate container the mixed preliminary product dwells for a lengthy period at high temperature, so that mixed-in additives can penetrate sufficiently into the raw material in order to guarantee good pressing.

In addition, a pulsating product input into a press or an expander can be avoided by means of the intermediate container.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail below in an example of embodiment with the aid of a drawing.

FIG. 1 shows a pelletising plant in a schematic representation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A pressing plant comprises a mixer 2, a dwell-time unit 3 (long-time conditioning), which are disposed above one another. Mixer 2 and dwell-time unit 3 each comprise identical front plates 5 and 6 respectively.

The conveying of the product takes place via suitable product supply opening 7 and product discharge opening 8. The transfer of the product into intermediate container 4 takes place via product discharge opening 8 of dwell-time unit 3 and from said intermediate container via a double screw 9 into a pellet mill 1.

The mill is fed with product to be pressed in a pulsation-minimised manner from intermediate container 4 by means of double screw 9, the screw flights whereof are mechanically coupled offset by 180° with respect to one another. A defined offset is essential in order to minimise the pulsing. The coupling can take place by means of a chain, gearing, belt etc.

Mixer 2 comprises a high-speed rotor for the intensive product mixing with vapour, gases and/or liquids. Dwell-time unit 3, on the other hand, is provided with a slow-running rotor. In dwell-time unit 3, the heated, mixed product is already held at a defined temperature for a preset time.

In mixer (2), the mixture is heated by means of vapour or other energy carrier up to 125° C. Furthermore, water for achieving the optimum pelletising water content and other additives for optimising the pelletising process are added in mixer 2.

An intermediate container 4 (a buffer depot) is disposed downstream of dwell-time unit 3. After the product in mixer 2 has been brought to temperature and the optimum water content, the product dwells for a defined dwell time in dwell-time unit 3, the conditioning temperature being held during the previously defined dwell time. This is achieved by means of insulation or external heating mats on dwell-time unit 3. First In-First Out applies.

Intermediate container 4 extends the dwell time and counteracts pulsating feeding of the mill, i.e. compensates for the latter.

The walls of intermediate container 4 and, if need be, dwell-time unit 3 are insulated or provided with external heating mats.

Mixer 3 and dwell-time unit 4 comprise tightly sealable openings in the respective jacket. Jackets and transfer openings as well as face plates 5, 6 can if necessary be fully lined with heating mats and insulating means. The inner surfaces of the jackets, including the product transfer openings (7 and 8), are preferably completely smooth.

Mixer 2 and dwell-time unit 3 contain rotors with work elements. These are designed screw-shaped (solid screw) in dwell-time unit 3 and paddle-shaped in mixer 2.

All the components are preferably compatible and combinable.

Dwell-time unit 3 has lower speeds of the rotor than mixer 2, so that the capacity of the individual units can be matched for a continuous operation.

After leaving pellet mill 1, the pellets are cooled in a cooler and then fed in the usual way to further stations.

Advantages of such long-time conditioning are:

-   -   steady, optimised pelletising process     -   high throughput     -   low energy expenditure per tonne of pellets     -   longer service lives of machine parts and wearing parts     -   better pelletising capacity for products difficult to pelletise

REFERENCE NUMBERS

1. pellet mill/press

2. mixer

3. dwell-time unit

4. intermediate container

5. face plate

6. face plate

7. product supply opening

8. product discharge opening

9. double screw 

1. A method for producing pressed products from fiber-containing raw materials such as wood, straw, peat or the like in a pressing or expander plant, the method comprising: mixing fiber-containing raw materials with water, vapor and, if need be, further additives while heating the raw materials and additives, wherein the mixture is held at high pressure for a dwell period before pressing.
 2. The method according to claim 1, wherein the mixture is held in an intermediate container during the dwell period.
 3. The method according to claim 1, wherein the conditioning temperature comprises temperatures up to 200° C.
 4. The method according to claim 1, wherein the mixture is preferably held at a temperature up to 150° C.
 5. The method according to claim 1, wherein fat and/or lignin are additionally added to the raw materials.
 6. The method according to claim 1, wherein the conditioning takes place pressure-less or in the presence of excess pressure.
 7. A device for producing pressed products from fiber-containing raw materials such as wood, straw, peat or the like, the device comprising: a pressing or expander plant performing a method according to claim 1, wherein an intermediate container is disposed downstream of a dwell-time unit following a mixer.
 8. The device according to claim 7, wherein the intermediate container is disposed upstream of a pellet mill or an expander.
 9. The device according to claim 7, further comprising a double screw disposed downstream of the intermediate container, wherein screw flights of the double screw are mechanically coupled offset by 180° with respect to one another.
 10. The device according to claim 7, wherein at least the intermediate container is thermally insulated. 